DISAPPEARANCE OF FLOW AND THE IN-MEDIUM NUCLEON-NUCLEON CROSS-SECTIONFOR ZN-64-27 COLLISIONS AT INTERMEDIATE ENERGIES(AL)

Light charged particles emitted from the reactions Zn-64+Al-27 at beam energies 49, 57, 69 and 79 MeV/u have been measured with a 4 pi multi detector array. The collective transverse momentum in the reaction pla ne (in-plane flow) is observed to vary strongly with the impact parame ter and the emitted fragments. The flow parameter decreases monotonica lly from 49 to 79 MeV/u and falls to zero at a beam energy about 79 Me V/u in central collisions. This energy of vanishing flow increases wit h impact parameter and may be the same for different fragments. The di sappearance of flow is also studied with the Boltzmann-Uehling-Uhlenbe ck equation and it is demonstrated that the flow parameter and E(VF) a re very sensitive to the in-medium cross section sigma(NN) and weakly sensitive to the nuclear compressibility in central collisions. Compar isons with experimental data indicate that sigma(NN) is around 35 to 4 5 mb in central collisions.